Decisions have been made by the European Union to ban environmentally hazardous substances in the near future; such decisions having been made with regard to end-of-life vehicles ELV, indicating that hazardous substances such as lead should be banned. Lead-based products, e.g. lead-based solder materials used for die or semiconductor chip attachment, will be banned and removed from the market in the near future.
Suitable alternative solder materials will in future be selected based on their economic viability. The cost of suitable alternatives would have to be at least comparable to that of current standard solder materials. Suitable alternatives will further have to meet the requirements and have the necessary properties to be used as a connection element, e.g. a solder connection. Such alternatives would have to be compatible for use on various surfaces, e.g. on lead frames or on chip back sides. They would also have to be electrically and thermally conductive, and robust and reliable enough for their application, e.g. being subjected to high temperatures or varying temperature loads.
A further technical requirement is that the solidus temperature of the solder material should lie above 260° C., so that the solder material will not melt and/or soften when subsequent processes are carried out, e.g. when soldering the printer circuit board. Further requirements of alternative solder materials are that they meet the requirements of ductility such that solder wires may be provided from the solder materials.
Up till now, the semiconductor field has not had a lead-free soft solder alternative for the connection of a chip to a lead frame, or from a clip to a bond pad, which may be achieved in mass production. The technical challenge lies in finding a lead-free solder which has a melting temperature over that of the solder material used in printed circuit boards e.g. Sn—Ag—Cu systems, with typical melting temperatures of 260° C. However, the melting temperature should not be too high either, as high mechanical stress would have to be installed in the system to cool down and at the same time, solidify the solder.
A lead-free solder material, apart from the melting temperature requirements, should have good wettability with various metallic surfaces e.g. chip surfaces or lead frames which may be used, to ensure that an optimal connection is provided. The solder material should further possess a certain ductility so that it can be produced and handled in wire form. That is, the solder material in wire form should not be brittle. The solder material has to withstand repetitive melting and solidification conditions, and mechanical and thermomechanical loads which may be applied to the material, without succumbing to degradation.